what's actually shaking is the contents of your eyeballs. the mounting of the lens is (which is why you can focus your eyes) and will pick up the vibrations in the crunch. my guess is that those vibrations are out of phase with refresh rate on your monitor, so the picture seems to jump up and down. (the frequency of the jumping should be equal to the differnece between your eye-parts vibration and the refresh rate, or of related vibrational harmonics.)

btw, when you see a monitor from a distance and it seems to have a slow wave of brightness moving down the screen, it's the same effect, only the interference is between the steady flickering of the lights caused by alternating current and the monitor refresh rates.

http://www.plus2physics.com/optical_interference/study_material.asp

study assignment: find cookies of different crunchiness and evaluate the difference in fluctuation frequency. be sure to conduct sufficient trials to ensure statistical validity.

These two tricks work best if your monitor is displaying a mostly white screen:

Second-Favorite monitor trick:
Hold a pencil by the eraser, straight up and down,and quickly waggle it back and forth. It will look as if it's made out of rubber, bending this way and that. The faster the waggle, the greater the bend.

Top favorite trick:
Get a can of freeze-spray or dusting spray (you will have to hold the dusting spray upside down to get the liquid to spray out.)

Gently squeeze the trigger and you can see the individual drops spraying out.

Side note: if you allow these drops to fall on the palm of your hand it feels...interesting. Be careful: a too-vigorous spray can cause frost-bite.

Hand your victim a bowl of the harder sort of corn chips while you're watching television. Whenever s/he takes a bite, ask "How long has your TV been doing that?" If you want to, you can offer to fix the set ... You just pretend to fiddle with the set until the bowl of corn chips has been eaten, then declare victory!